Fatigue is a very familiar problem in everyday life. Many modern people who are under a lot of stress suffer from various kinds of fatigue. However, scientific and/or medical studies relating to “fatigue” have only been fragmentarily conducted, and hardly any studies have been done on decisive means or quantitative standards for quantitatively and objectively expressing “fatigue”, which is a subjective symptom.
So far, muscle fatigue (exercise fatigue) has been mainly studied as a representative example of “fatigue”. The indicator focused on in this case is an increase in the production of lactic acid in muscles. However, lactic acid is basically an important energy source for the cranial nervous system, and the theory that it inhibits muscle activity is now negatively perceived. In addition, during muscle fatigue, the phenomena of increase of pyruvic acid and drop of pH value in body fluids are known to occur. These phenomena are indeed observed when a certain stress, a load to muscle (exercise load), is given; however “fatigue” is different from local muscle exhaustion and is considered to be a broader and larger physiological phenomenon that manifests in the living body.
Patent Document 1 discloses a method for quantifying stress by using as indicators concentrations of adrenal sex steroids and their metabolites in saliva. Patent Documents 2 and 3 disclose a method for assessing the anti mental fatigue activity of test substances by measuring amino acids such as taurine, leucine, and isoleucine in blood. Although these documents disclose methods for assessing stress or fatigue by using biological factors in body fluids as indicators, the quantification of fatigue in everyday life is yet to be achieved.
Immune strength is thought to deteriorate when humans are fatigued, and virus infection can be given as one form of expression of this deterioration of human immune strength. However, a relationship between fatigue and virus infection in humans has not been elucidated.
Eight species have so far been identified as herpesviruses that mainly infect humans. Herpesviruses are large DNA viruses. They are classified into three subfamilies α, β, and γ, mainly according to the phylogenetic tree of evolution, and each subfamily has common biological characteristics. For instance, α-herpesvirus is neurotropic and undergoes latent infection and reactivation in nerve cells, and γ-herpesvirus has tumorigenicity.
Characteristics common to all herpesviruses include establishment of latent infection by viruses hiding within the body following mainly childhood infections, and recommencement of growth following some stimuli (reactivation).
Fatigue, especially mental fatigue and stress, is cited as a possible stimulus that induces reactivation. Among herpesviruses, herpes simplex virus type 1 is the causative of labial herpes. The reactivation of this virus is often observed as manifestation of labial herpes, which happens in many cases when virus carriers become extremely exhausted.
However, it actually remains almost unknown as to the sort of stimulus that induces the reactivation of herpesviruses. The relationship between the reactivation of herpesviruses and fatigue/stress has not been scientifically proven, and the above-mentioned relationship between herpes simplex virus type 1 and fatigue is merely a folk belief.
As mentioned above, methods for objectively judging muscle fatigue (exercise fatigue) due to muscle load (exercise load) have been proposed. However, hardly any reports exist regarding an objective method for assessing fatigue symptoms of everyday life although many modern people are experiencing them. Fatigue symptoms of everyday life, if they are ignored, carry the risk of directly resulting in death from overwork, a sudden death caused by long-term overworking. Although the problem of overwork death is recognized as very important from medical, economic, and social standpoints, scientific mechanisms underlying it are hardly clarified. Therefore, an objective method for assessing fatigue level is required in order to prevent overwork death, which has recently become a social problem.
Furthermore, since most medicines, health foods, or such, including nutrition-supplement drinks that are flooding the market, advertise fatigue recovery or prevention functions, scientific evidence regarding the functionality is widely required not only by consumers, but also by the market and society as a whole.
As mentioned above, knowledge on fatigue caused by exercise load is present. However, fatigue caused by exercise load and fatigue in everyday life are completely different from each other, and a method for assessing fatigue in everyday life has not been developed. Thus, the development of a simple, easy, and objective in vivo method for assessing fatigue in everyday life and their methods of application are strongly required.
[Patent Document 1]
    Japanese Patent Application Kokai Publication No. (JP-A) Hei 11-038004 (unexamined, published Japanese patent application)[Patent Document 2]    JP-A Hei 11-304792[Patent Document 3]    JP-A Hei 11-304793